That's a fair argument, but it's a matter of trade-offs. All that bartering means that no-one's program will be actually enacted, and when things don't go well

That's a fair argument, but it's a matter of trade-offs. All that bartering means that no-one's program will be actually enacted, and when things don't go well there is nobody to take the actual responsibility - including voters, because everybody can always say "well but this wasn't what the party I voted for was proposing" - even if that party got a relative majority. Personally, I favor a system where there is a clear winner chosen by the majority of popular vote, but where minorities have a "right to representation" where they can make their voices heard in the political debate. Not easy, but possible.